Delete most of old theme mixin

What does this MR do and why?

Delete most parts of the old theme mixin that styled the old navigation.

Part of &11875.

Screenshots or screen recordings

No visual changes in the new nav. The old nav isn't available anymore.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.